Without limiting the scope of the invention, its background is described in connection with lithium ion batteries.
Generally, lithium ion batteries transport lithium ions between the anode and cathode with the simultaneous oxidation or reduction of the host electrodes, respectively. Cathode materials common in the art include transition metal oxides containing lithium, e.g., layered lithium cobalt oxide (LiCoO2), spinel lithium manganese oxide (LiMn2O4), and olivine lithium iron phosphate (LiFePO4). For example, lithium ion batteries use layered lithium cobalt oxide cathodes; however, these materials are expensive and environmentally unfriendly due to their cobalt content. As a result, alternative materials are being developed as electrodes that have the desired discharge capacity, which is related to the amount of lithium that can be reversibly extracted, and discharge voltage, which depends on the transition metal ion and crystal structure.
For example, common electrode materials include spinel LiMn2O4 and olivine LiFePO4 that include Mn and Fe respectively, and hence are inexpensive and environmentally benign. However, the spinel LiMn2O4 cathode has been plagued by severe capacity fade at elevated temperatures.1-7 The spinel electrodes are unstable in the cell environment, and particularly unstable when operated at temperatures above room temperature.
The capacity fade is generally thought to be due to the dissolution of manganese from the lattice into the electrolyte and then into the carbon anode. Alternative spinel compositions achieved through cationic substitutions have been pursued, but they could not completely overcome the capacity fade problem.
In addition, the process of synthesizing the spinel structure and chemical substitutions could result in local defects and microstructural differences that could influence the electrochemical performance factors including capacity retention, rate (power) capability, and storage characteristics.
For example, U.S. Pat. No. 5,674,645 (the '645 patent) entitled “Lithium Manganese Oxy-Fluorides for Li-Ion Rechargeable Battery Electrodes” issued to Amatucci, et al. on Oct. 7, 1997. The '645 patent discloses that the cycling stability and capacity of Li-ion rechargeable batteries are improved by the use of lithium manganese oxy-fluoride electrode component intercalation materials having the general formula, Li1+xMyMn2−x−yO4−zFz, where M is a transition metal, e.g., Co, Cr, or Fe.
Similarly, U.S. Pat. No. 6,087,042, entitled, “Positive Electrode Material for Secondary Lithium Battery” issued to Sugiyama, et al. discloses a positive electrode material for a secondary lithium battery excellent in high temperature cycle characteristics which is a lithium manganese oxyfluoride having a spinel structure, wherein the oxyfluoride has a composition represented by the composition formula Li1+xMn2−xO4−yFz: wherein x represents a number from 0.0133 to 0.3333; y represents a number from 0 to 0.2 (exclusive of 0); and z represents a number of from 0.01 to 0.2 (exclusive of 0.01), with the proviso that (y−z) is more than 0 but not more than 0.07. The positive electrode material for a secondary lithium battery of is said to exhibit not only a high cycle durability of charge/discharge but also a minimum drop of a charge/discharge initial capacity to provide a high energy density.